1. Field of the Invention
The present invention relates to a laser radar device.
2. Description of the Related Art
In order to find a vehicle ahead or an obstacle on a track, or to detect a white line, which is used as a lane marking on a road, or a road stud such as a cat's-eye, in general, an object type determination device is used which employs a scanning laser radar. A laser radar device emits a laser beam ahead of a vehicle, and receives reflected light, thereby detecting an obstacle or the like ahead of the vehicle.
A generic laser radar device includes a photoemitter that generates a pulsed laser light beam, and that scans the light beam in a horizontal direction; a photodetector such as a photodiode that receives the laser light which is reflected by the object in front of the vehicle, and that converts it into a voltage signal; and an optical receiver including an optical element such as a lens that guides the reflected light toward the photodetector. A distance between the vehicle and the object can be calculated by a time difference between a moment at which the pulsed laser is generated and a moment at which the reflected light is received by the photodetector.
Usually, a pulsed laser light is generated by applying a pulse-shaped driving current to a semiconductor laser diode, and the pulsed laser light is guided to an optical scanner that scans light in the horizontal direction through optical coupling element such as a coupling lens. For example, a polygon mirror or a galvanometer mirror has been used as the optical scanner. By scanning a laser beam by a mirror device, distance measurement can be performed across a wide range in the horizontal direction.
In addition to expansion of the range of the measurement in the horizontal direction, recently, there is a need for two-dimensional scanning, in which a beam is also scanned in a vertical direction, and there is a need for multi-line scanning (multi-layered), in which a measured area in the vertical direction is divided and a light beam is horizontally scanned.
As a means for achieving the two-dimensional scanning or the multi-line scanning, in general, a configuration can be considered in which scanning devices, scanning directions of which are different by 90 degrees, are connected in series, and beam scanning in the horizontal direction is performed and subsequently the beam scanning in the vertical direction is performed. As a means for more easily achieving the multi-line scanning, a method can be considered in which a tilt angle is defined for each of reflection planes of a rotational polygon mirror relative to a rotation axis of the rotational polygon mirror such that the tilt angles of the corresponding reflection planes are different from each other. Additionally, as a means for enabling the multi-line scanning in which measured areas are different in the vertical direction, a configuration may be adopted in which more than one of photodetectors are arranged in the vertical direction.
As described above, for a laser radar device which scans a laser beam, a method has already been known in which, in addition to scanning in the horizontal direction, measurement is performed by dividing a detection area in the vertical direction.
Unfortunately, with a laser radar device according to related art, as a number of layers in the vertical direction increases, the cost is significantly increased and/or the size of the device is significantly increased.
For example, when the configuration is adopted in which the plural photodetectors are arranged in the vertical direction, it may be required to prepare the photodetectors so that a number of light receiving areas of the photodetectors is equal to a number of layers of the detection area, which is divided in the vertical direction. For a laser radar device for a medium to long distance which is greater than several tens of meters, in general, an avalanche photodiode (APD), which has a high detection sensitivity, is used as the photodetector. However, an APD element is expensive. When the number of the light receiving areas is increased, the number of the APDs to be used is also increased, thereby increasing the cost. In addition, for the divided detection areas, it may be required to individually prepare detection signal processing systems, such as an amplifier circuit, and control is complicated. This can also be a cause of an increase in the cost.
When the detection area in the vertical direction is divided by making the tilt angles of the mirrors included in the polygon mirror relative to the rotation axis to be different from each other, it may be required that a number of the mirror surfaces is greater than or equal to a number of the layers in the vertical direction. Thus, the size of the polygon mirror is enlarged. A problem is that it can be a cause of an increase in size of the whole device.
Patent Document 1 (Japanese Unexamined Patent Publication No. H09-274076) discloses a configuration of a laser radar device which scans a laser beam. In the configuration, beam scanning is performed by a polygon mirror, which includes mirror surfaces whose tilt angles relative to a rotation axis of the polygon mirror are different from each other, so as to divide a detection area in the vertical direction into multiple layers.
The invention disclosed in Patent Document 1 includes a configuration such that the multiple areas, which are divided in the vertical direction by the polygon mirror having the mirror surfaces with the different tilt angles, are scanned by a beam. However, with the invention disclosed in Patent Document 1, when the number of the layers in the vertical direction is increased, it may be required to prepare the number of the mirror surfaces which is greater than or equal to the number of the layers. Thus, the invention disclosed in Patent Document 1 may not solve the problem in which the size of the device may be enlarged.